Balanced pneumatic conveying systems for pulverulent material



2,814,531 EMS 1957 R. M. MURRAY, JR

BALANCED PNEUMATIC CONVEYING 'SYST FOR PULVERULENT MATERIAL Filed Aug. 27, 1956 Receivin Tank INVENTOR. FAY M. Maze/W J2 BY ijnited States Patent BALANCED PNEUMATIC CONVEYING SYSTEMS FOR PULVERULEN T MATERIAL Ray M. Murray, Jr., Seattle, Wash., assignor to Clough Equipment Co., Seattle, Wash., a corporation of Washington Application April 27, 1956, Serial No. 581,144

11 Claims. (Cl. 302-22) period normal operations at the plant must proceed, and it is quite likely that the users tank will be drawn upon for supplies of sugar while the delivery operations is in progress.

The draw-ofif valve of such tank is of a type that opens away from the tank, with open clearance all around. It has been found that when such an open clearance drawoif valve is opened while a delivery is progressing, any positive pressure within the receiving tank may tend to blow the sugar too rapidly past the draw-01f valve, and to blow dust about the vicinity of the tank, while any negative pressure within the tank will impede withdrawal. Especially are such pressure differences, and the undesirable effects thereof, likely to occurvif the supply in the tank being drawn upon is low. The sugar dust is objectionable, in addition to the economic loss involved, and

in an attempt to avoid the results indicated, various 'expedients have been employed, but heretofore it has been found impossible to avoid some fluctuations of pressure within the receiving tank, of a value which, being applied to the sugar at the draw-off valve thereof, causes expulsion of dustinto the atmosphere in the vicinity of the tank, if positive, or slowing down or halting of delivery, if negative.

. It is the object of the present invention to provide a pneumatic system or mechanism for-effecting such a delivery, incorporating provisions which will automatically, and at alltimes and under substantially all conditions, maintain for all practical purposes zero pressure (with relation to atmospheric) within the receiving tank, to'the end that there is no pneumatic pressure or force active at the draw-off valve of such a tank tending to cause expulsion of dust when the valve is opened for withdrawing sugar during the time that a delivery of sugar is being efiected, or contrariwise, to cause variation in the withdrawal rate.

It is also an object to provide a pneumatic mechanism of the character indicated, which can be mounted substantially wholly upon a transport vehicle, such as a truck or trailer, ready for connection to the receiving tank installed Within a building, and which receiving tank needs only a receiving conduit and an air return conduit to which like conduits upon the vehicle need to be connected.

A still further object is the provision of means in such a pneumatic mechanism by which expulsion of sugar dust about the vehicle or at any point in the system is avoided, notwithstanding the employment of certain valves and orifices which are open or may at'times open freely for communication between the external atmosphere and the interior of the pneumatic system.

The invention will be more fully understood as this specification progresses, and the principles thereof which are the subject of this invention will be defined in the appended claims. The drawing illustrates purely diagrammatically the complete system, including that part which is installed in the users building, and that part with which this invention is particularly concerned, and which is mounted upon the transporting vehicle.

While as has been indicated above, the system of this invention has been designed primarily to serve the purpose of delivering sugar, this is so principally because sugar is one material which is delivered in this manner in large quantities to numerous users, but the same principles may be employed, of course, in the delivery of similar materials, and the invention is not to be understood as limited in any manner to the delivery of the particular material (sugar) which has been referred to by way of illustration.

The users building or plant is indicated in the drawing by the dash-line box U, and the transporting vehicle by the similar dash-line box V. Within the building it is assumed there will have been installed a receiving tank 9 with a draw-01f valve in its bottom, capable of being opened and closed at will by an actuator 95. A receiving conduit 91 leads from a coupling 92 to the upper space 99 within the tank 9, and an air return conduit 93 leads from the space 99 to a coupling 94. The two couplings, it will be assumed, are conveniently accessible from a driveway at the exterior of the building U. The pneumatic mechanism which constitutes the present invention is for use in delivering the pulverulent material to such an installed system as has just been described, and it is not considered that the installed system in the building U is part of the present invention, except as an element in the whole combination.

Mounted uponthe transport vehicle V is a transporting tank 1, a blower 2, and a separating chamber 3, together with suitable conduits connecting them, and valvecontrolled or freely open orifices in such conduits, as will be described hereinafter. The separating chamber is divided by a filter 31 into two separate spaces, designated A and B to distinguish them. It will be remembered that the drawing is purely diagrammatic, hence the various elements of the system are not shown relatively proportioned as they would be in actual use.

A delivery conduit, generally designated by the numeral 20, leads from the outlet of the blower 2 to and past the outlet, usually at the bottom, of the transporting tank 1, and thence to the coupling 92, by which it is secured to the receiving conduit 91 for the purpose of making a delivery. A return conduit 30 leads from the coupling 94 to the intake of the blower 2, but in advance of the blower there is installed the separating chamber 3. The return conduit 30 leads into the space A of the latter, and from the space B to the blower inlet, so that air which is returned to the blower is filtered at 31 to remove any sugar dust that may be carried in the air. The filtered-out dust is deposited in the bottom of the space A, and is removed from time to time.

Such a system would normally, but not necessarily, incorporate an equalizing duct 10, leading from the delivery conduit 20, in advance of the latters connection to the bottom of the transporting tank, and discharging into the top of the transporting tank 1, above the contents thereof. Valve means, diagrammatically indicated at 11, control the rate of gravity discharge of sugar from the transporting tank 1 into the delivery conduit 20, where the moving air as a vehicle picks it up for delivery by way of the receiving conduit 91 into the upper space 99 of the receiving tank 9. The pressure admitted to the upper portion of the transporting tank 1, by way of the equalizing conduit 10, serves to eliminate any minus pressure differential above the material in the tank 1, so that the material will run freely into the delivery con- 3 duit 20, in such quantities as are controlled by the valve 11.

The air, after delivering its burden of sugar to the receiving tank 9, must be removed in order to avoid the building up of back pressure. Clearly, it can not be discharged into the open, for it will entrain some sugar dust, which must be filtered out. It has been found most practicable to return all air from space 99 to the blower 2 by way of the air return. conduit 93 and the connected return conduit 30 on the transport vehicle V. The entrained sugar dust. is separated from the air before it reenters the blower 2, and the separating chamber 3 functions to accomplish this, in the manner already described, the filter medium 31 being of a character to accomplish this end.

At the begiiming of a delivery the filter medium 31 will be clear, and will cause a minimum pressure difference between spaces A and B. The system is so designed that the pressure drop within the delivery side of the closed circuit equals the pressure drop within the return side thereof, at the initiation of a delivery, leaving pressure Within space 99 equal to Zero, with respect to atmospheric. As the delivery proceeds, filtered outsugar dust will tend to collect upon the filter medium 3i, and to clog its air passages. The pressure difference as between spaces A and B increases, and it is this change in the pressure drop in the return side which reflects throughout the pneumatic system and causes changes in pressure within any given space, and particularly in space .99. It is necessary to maintain the pressure within the space 99 automatically at atmospheric, to avoid blowing out, or the reverse, when the draw-E valve 96 is opened, and it is this object to which the present invention is primarily directed.

The total mass of air in the pneumatic system as a whole, when all parts are connected but the system is quiescent (with blower 2 inactive), may be taken as represented by M. When the blower begins to operate, the systems air mass increases, to M +m, and the increment m must be drawn into the system. A balancing valve 4 is provided for the purpose; it is normally closed by a light biasing spring 40 or the like. This balancing valve 4 must be located in the minus pressure portion of the pneumatic system, in advance of the blower intake, and since it opens inwardly it will not permit escape of dust-laden air, and can be and should be located in advance of the filter medium 31; it is preferably located at the separating chamber 3, in communication with the space A.

Not only is the balancing valve 4 intended to admit airto augment the total air mass within the system whenever the blowcr 2 starts up, but it is intended to maintain the pressure within space 99 from exceeding atmospheric. There is a pressure drop in the connected return conduits 93, 30 between space 99 and the space A, the value of which during normal operation depends upon the design, and is not subject to change by transient conditions during operation. The biasing means 40 is set at a value to permit opening of the balancing valve 4 whenever ,the pressure drop in this part of the system tends to exceed the designed pressure drop, as it does while filtering progresses, and it acts quite sensitively, with the result that if pressure within space 99 tends to drop below atmospheric (which would cause an increase in the pressure drop) the balancing valve 4 opens, and the pressure drop is restored to normal, and pressure within space 99 does not drop below atmospheric.

Should pressure in space 99,tend to increase above atmospheric, as it might, for instance, because of buildup of sugar collected upon the filter medium 31, and consequent increase of pressure in space A, such increase of pressure in space A would tend to decrease the pressure drop between 99 and the separating chamber 3. Clearly such pressure build-up can not be relieved through the normally closed, inwardly opening balancing valve 4. An outwardly opening but normally closed relief valve 5 is provided to relieve any such pressure build up. It too is located in the minus pressure part of the system, and because it would be undesirable to permit air which carries a burden of sugar dust to escape to the atmospheric, the relief valve 5 communicates with chamber B of the separating chamber, at the clean air side of the filter medium 31. Just as any increase in the pressure drop between 99 and 3 automatically opens balancing valve 4 to maintain pressure in 99 at atmospheric, so any decrease in the pressure drop between 99 and 3 opens rclief valve 5, and restores the designed pressure drop, maintaining pressure in 99 at atmospheric.

As has been pointed out, the system requires an increment In to the contained air mass M upon starting the blower. Upon shutting down the blower this increment must be eliminated. It is expelled through the relief valve 5 automatically. Slowing down of the blower tends to produce a back pressure ahead of its intake, and the relief valve 5, at this location, vents the excess.

In essence, then, by any tendency for pressure in space 99 to rise above atmospheric causes relief valve 5 to open, preventing such rise, and any tendency for pressure in space 99 to drop below atmospheric causes balancing valve 4 to, open, preventing such drop. The changing pressure drop across the filter medium 31 will tend to affect the pressure in space 99, as a delivery operation progresses, but the arrangement described automatically maintains pressure within 99 at atmospheric at all times. It follows that opening of the draw-off valve during a delivery operation-can have no effect upon the pressure in space 99.

The biasing force at 40 can be variable, so that the system as a whole can be adapted to different pressure drops between 99 and 3, arising for example from differing characteristics of individual air return conduits 93, or of individual receiving conduits 91, in different plants.

In order to avoid accidental addition of excess air, particularly to the pressure side of the system within the conduits 20 and 91, or to compensate for fluctuations in the operation of the blower 2, and in order to bleed off any increment in drawn in at balancing valve 4 during normal operation, a small orifice 22 is provided in the delivery conduit 20, intermediate the blower outlet and any connection to the transporting tank 1. The size of this on'fice 22 is so regulated or designed that a small amount, a minor fraction of the air flow developed by the blower 2, is continuously bled off, and any momentary excess will bleed out here, rather than affecting the total mass of air within the system. The presence of the orifice 22 eliminates opening of the relief valve 5 under the influence of transient fluctuations of pressure; the relief valve 5 opens only to vent so much air as is necessary to prevent a pressure rise in space 99.

A non-return valve 23 is desirably installed in the delivery conduit 20, between the orifice 22 and any connection to the tank 1, and a safety valve 24 is also installed in this portion of the delivery conduit, the same being biased, as by the spring 25, to remain closed so long as the normal operating pressures within this part of the delivery conduit are not exceeded. It does not function so long as the delivery is proceeding normally, but is provided in order to avoid the possibility of damage in the event of clogging or malfunctioning of the system.

With the system at rest and the blower 2 inoperative, all pressures are zero with reference to the atmosphere. When the blower 2 is started, but with the valve 11 closed, such air as is required to be added to the system to create a pressure in the delivery conduit 20 and the receiving conduit 91 is admitted by way of the balancing valve 4. Now when the pulverulent material is admitted past the valve 11 to the pneumatic system, that is, into the delivery conduit 20, the blower discharge pressure rises, compressing still more air into the transporting tank 1 byway of the conduit 10. This additional mass of air is admitted by way of the balancing valve 4. During steady operation the pressure drop across the filter 31 rises, owing to the progressive clogging of the pores of the filter. Vacuum at the inlet to the blower 2 rises, but is countered by bleeding oft at 22, supplemented if necessary at 24. In addition the balancing valve 4 admits more air, and so prevents transfer of any pressure difference to the receiving tank 9. The balancing valve 4, as has already been stated, has its biasing means 40 set to almost exactly match the pressure drop in the return conduits 93 and 30, and so the pressure in the receiving tank 9 remains almost precisely zero.

As the transporting tank 1 empties or approaches the empty level, the pressure required drops, so that higher pressure air in the tank 1 escapes with the material past the valve 11. Nevertheless anymaterial or prolonged pressure rise in the receiving tank 9 is prevented by anticipating any such surge, or by the venting of the excess air at the relief valve 5, either or both. Finally, as has already been indicated, any accidental addition of excess air and a small portion of the blower pressure is relieved constantly through the orifice 22, and fluctuations of any large degree are relieved by way of the safety valve 24.

At no time during the cycle has the pressure in the receiving tank 9 departed substantially from atmospheric. Variable pressure drop across the filter 31 is compensated for on a completely automatic basis. All parts of the system are capable of being mounted upon and transported with the vehicle which supports the transporting tank 1, and require only the connection and closure of the system through the couplings 92 and 94.

I claim as my invention:

1. For use in delivering a pulverulent material to and maintaining zero pressure (with respect to atmospheric) within a receiving tank which is equipped with a receiving conduit, an air return conduit, and a draw-off valve in its bottom which is likely to be opened during the delivering operation: pneumatic mechanism comprising a blower, a closed transporting tank having a delivery opening, a separating chamber and a filter therein, a delivery conduit leading from the blower outlet to and past the transporting tanks delivery opening, and thence to a connection to the receiving conduit, a return conduit leading from a connection to the air return conduit to the separating chamber at one side of the filter, and from the separating chamber at the opposite side of the filter to the blower intake, a balancing valve opening from the atmosphere to the interior of the separating chamber, means biasing said balancing valve to close, but yielding to permit the same to open whenever a minus pressure, of a value exceeding the pressure drop in the connected return conduits, occurs within said separating chamber.

2. Pneumatic mechanism as in claim 1, including further a restricted orifice located in the delivery conduit intermediate the blower outlet and the connection to the transporting tank, of a size to bleed off constantly a minor fraction of the blower flow.

3. Pneumatic mechanism as in claim 1, including a normally closed relief valve communicating with the separating chamber and afiording escape to atmosphere upon the occurrence in the separating chamber of a plus pressure.

4. Pneumatic mechanism as in claim 3, wherein the balancing valve is located at the side of the filter nearer the receiving tank connection, and the relief valve is located at the opposite side of the filter.

5. Pneumatic mechanism as in claim 1, including a pressure equalizing duct leading from the delivery conduit, at a point in advance of its connection to the transporting tank, to the top of said transporting tank, above the contents thereof.

6. Pneumatic mechanism as in claim 1, including a nonreturn valve in the delivery conduit located intermediate the blower outlet and any connection between the delivery conduit and the transporting tank, and a normally closed safety valve leading from the delivery conduit at a point intermediate said nonreturn valve and the blower outlet, and biased to remain closed until the occurrence in the delivery conduit of pressures exceeding normal operating pressure.

7. Pneumatic mechanism for delivering a pulverulent material from a transporting tank to a receiving tank such as is equipped with a receiving conduit and an air return conduit, and with a draw-ofl? valve in its bottom which is likely to be opened during the delivering operation, which pneumatic mechanism comprises, in combination: a blower, a transporting tank having a delivery opening, a separating chamber and a filter therein, a delivery conduit leading from the blower outlet to and past the transporting tanks delivery opening, and thence to a connection to the receiving conduit, a return conduit leading from a connection to the air return conduit to the separating chamber at one side of the filter, and from the separating chamber at the opposite side of the filter t0 the blower intake, a balancing valve opening from the atmosphere to the interior of the separating chamber, at the same side of the filter as the connection from the air return conduit, means biasing said balancing valve to close, but set to yield and permit the same to open whenever a minus pressure, of a value exceeding the pressure drop in the connected return conduits, occurs within said separating chamber, a relief valve, normally closed, located in the separating chamber at the side of the filter opposite said balancing valve, to open upon the occurrence of a plus pressure in the separating chamber.

8. Pneumatic mechanism as in claim 7, including also a restricted orifice located in the delivery conduit intermediate the blower outlet and any connection to the transporting tank, of a size to bleed off constantly a minor fraction of the blower flow.

9. Pneumatic mechanism as in claim 8, including further a non-return valve in the delivery conduit located intermediate the blower outlet and any connection to the transporting tank, and a normally closed safety valve leading from the delivery conduit in advance of said nonreturn valve, and biased to remain closed so long as delivery conduit pressure does not exceed normal operating pressures.

10. A pneumatic system for deliverying granular material from a transporting tank to a receiving tank which is equipped with a draw-off valve, said system comprising, in combination, a blower, a delivery conduit extending from the blower outlet to and past the transporting tank, to receive such material from the latter, and thence beyond to the space within the receiving tank; a return conduit extending from the space within the receiving tank to the blower intake, to complete a closed cit-- cuit; a separating chamber interposed in said return conduit between the receiving tank and the blower intake, at such distance from the receiving tank as will produce a given pressure drop during normal operation; a filter medium in said separating chamber, dividing the same into two spaces A and B which communicate respectively with the receiving tank and with the blower intake; a balancing valve alfording communication between space A and the atmosphere, and biasing means normally retaining the balancing valve closed, but permitting the same to open whenever the given pressure drop, between receiving tank and separating chamber, tends to increase; a restricted orifice in the delivery conduit intermediate the blower outlet and any connection to the transporting tank, communicating with the atmosphere; and a normally closed relief valve alfording communication between space B and the atmosphere, arranged to open whenever the same given pressure drop tends to decrease.

11. For use in delivering a pulverulent material to and maintaining pressure equal to atmospheric within a receiving tank which is equipped with a receiving conduit, an air return conduit, and a draw-01f valve in its bottom which is likely to be opened during the delivering oper- 7 ation: pneumatic mechanism comprising a blower, a closedtransporting tank having a delivery opening, a separating chamber, a filter dividing the separating chamber into two separate chambers A and B, a delivery conduit leading from the blower outlet to and past the transporting tanks delivery opening, and thence to a connection to the receiving conduit, a return conduit leading from a connection to the air return conduit to chamber A of the separating chamber, at one side of the filter, and from chamber B of the separating chamber, at the other side of the filter, to the blower intake, to define a closed material-handling circuit, wherein the delivery side is pressure-balanced by design with the return side, to produce atmospheric pressure within the receiving tank at the initiation of the delivering operation, a balancing valve opening from the atmosphere to the interior of chamber A means biasing said balancing valve to close so long as the pressure drop in the return side of the closed circuit does not exceed the intended pressure drop, but opening automatically to compensate for any change in the pressure drop in such return side as material collects upon the filter, and a restricted orifice located in the delivery conduit intermediate the blower outlet and the connection to the transporting tank, of a size to bleed 01f air from the blower flow to compensate for any air admitted to the closed circuit by the balancing valve.

References Cited in the file of this patent UNITED STATES PATENTS 1,669,084 Grindle May 8, 1928 1,718,507 Wenzel June 25, 1929 2,234,788 Williams Mar. 11, 1941 2,539,109 Webb Jan. 23, 1951 

